I would assume that since the French could take a coastal route to St. Petersburg, there would be less attrition and a shorter route, and all the coastal cities such as Riga and Reval could be captured as well, and if the Russians burned everything down, then they could just go back to Poland and resupply?
Unless my history is off, I thought St. Petersburg was the capital of Russia from 1721 to 1918 or so, and 1812 is between those 2 numbers.
Napoleons goal was never to capture Moscow or St. Petersburg, his goal was to smash the Russian army to bits in one decisive battle as he had done at Austerlitz and Friedland. Through such a battle he could impose his will on Russia(such as the reentrance of the continental system and a independent Poland). Yet the Russian army never allowed such a battle to take place so Napoleon persued them and went to far, probably Napoleon never expected he would come as fas as Moscow. Hope this answers your question.
Edit:furthermore Napoleon followed the route to Moscow because that was the route the Russian army was taking. 2nd edit: sources "1812" Adam Zamosky and the most recent one "Napoleon the great" Andrew Roberts there are more sources I could name but these mainly outline the point I have made.